Downstream hydrogen plasma cleaning for mass metrology at NIST
On International Metrology Day, May 20, 2019, the redefinition of the International System of Units (SI) is scheduled to be implemented. When this occurs the kilogram, the last remaining unit defined by a physical artifact, will be redefined to be based on a fundamental physical constant, the Planck constant. As a consequence, the NIST mise en pratique, a series of steps for the realization and dissemination of the revised definition of the kilogram, must be significantly modified. In particular, the mise en pratique will need to address for the first time the measurement and handling of masses in a vacuum. At NIST this will includes 5 major components: NIST-4 Kibble Balance, Magnetic Suspension Mass Comparator (MSMC), Mass-in-Vacuum Comparator, storage chambers, and mass transport vehicles (MTV). In addition to these components, there also is a need to develop new procedures to maintain and clean masses in vacuum. Over long periods of time masses can become coated with a carbon containing contamination layer, even while stored in a vacuum. A downstream hydrogen plasma source is being investigated as a means of gently removing these carbonaceous layers. Atomic hydrogen can react with carbon on the mass surface and convert it to volatile hydrocarbons which can then be pumped out of the system. Initial results demonstrating the etching of carbon coatings off a quartz crystal microbalance (QCM) in our downstream hydrogen plasma cleaner will be presented.